Boron nitride is a thermally stable, highly refractory material of increasing commercial significance. Typically, boron nitride is produced by processes wherein boric acid is utilized as the boron source of reaction compositions. Suggested processes for producing boron nitride from boric acid are described in U.S. Pat. Nos. 2,922,699; 3,241,918; and 3,261,667 as well as in British Pat. Nos. 874,166; 874,165; and 1,241,206. JP Patent Publication No. 06-040713 discloses a process for producing boron nitride from colemanite.
In these prior art processes for making boron nitride, borate starting material containing alkali/alkali earth metal compounds—particularly sodium and calcium compounds—when purged with ammonia at temperatures of 1200° C. or higher, form boron nitride plus by-products that require additional washing/treatments steps to recover boron nitride of high purity. Some of the by-products are various forms of calcium borate, which are removed from the boron nitride by leaching with hydrochloric acid. Others processes use a de-ionized water wash to purify the boron nitride. U.S. Pat. No. 3,415,625 discloses a continuous or batch process for a boron nitride product of high purity after a washing/treatment step.
U.S. Pat. No. 4,045,186 discloses the use of Li3N to react with small particle-sized boron nitride for the subsequent recrystallization of larger-sized crystalline hexagonal boron nitride from the mixture at elevated temperatures of greater than 1100° C. China Patent Publication No. CN1539729A discloses a process for preparing boron nitride from boron trifluoride ether and lithium nitride by solvent heat synthesis method. German Patent Publication No. DE4108367C1 discloses a process to prepare boron nitride with mainly a hexagonal structure that is comprised of reacting a suspension of lithium nitride (in anhydrous di(2-6C)alkyl ether) with an excess of trifluoroborane di(1-6C) alkyl etherate (pref. dibutyl etherate) with stirring, at 20-230° C. for 2-24 hours.
In addition to the washing requirement and before the washing step, the prior art processes typically require a two-step approach to make boron nitride crystals. The first step is a calcination step, wherein the reactants are heated to a temperature of up to 1100° C., forming an incompletely reacted boron nitride in the “turbostratic” form. In the second sintering step, the turbostratic boron nitride is heated to a temperature of 1500 to 2300° C. to control the crystallinity and purity of the final boron nitride product.
The invention relates to a process for preparing boron nitride of high purity and good yield. Additionally, in the process of the invention, the calcination and sintering steps are combined into one single firing step, wherein the metal borate impurities vaporize away forming high purity boron nitride crystals. Lastly, the invention relates to an improved process wherein the washing/leaching of the reaction product is optional for a boron nitride of high purity.